This invention relates to a substrate system for spray forming and an associated method.
Spray forming, also referred to as spray casting, of molten metal is a fairly well known process for producing various types of metal products. Spray forming consists of introducing a controlled stream of molten metal into a gas-atomizing nozzle where it is impacted by high-velocity jets of gas, usually argon or nitrogen. The resulting spray of metal droplets is deposited onto a substrate to form a highly dense product. See U.S. Pat. Nos. 4,938,278; 5,110,631; 5,143,140; and 5,154,219.
Various types of substrate problems exist in spray formed products. For example, one problem concerns a high degree of substrate layer porosity which is formed due to high chilling at the substrate layer at the leading edge of the spray. U.S. Pat. No. 4,917,170 recognizes the problem of substrate layer porosity and attempts to solve the problem by providing a low thermal conductivity substrate for use in the spray forming process. The substrate is made entirely of a low thermal conductivity material. U.S. Pat. No. 5,240,061 also recognizes the problem of substrate layer porosity in spray forming processes. A moving substrate in the form of a thin foil is provided to reduce and limit heat extraction from the particles being deposited on the substrate so as to reduce substrate layer porosity.
Another problem relates to inadequate heat extraction from a bulk layer of the sprayed metal droplets deposited on the substrate. Typically, slow cooling rates in the bulk layer results in coarse constituent particles and large grains. As can be appreciated, this problem is enhanced by substrate designs aimed primarily at resolving the problem of substrate layer porosity. In other words, a substrate system which reduces the problem of substrate layer porosity enhances the problem of inadequate heat extraction from the bulk layer, while a substrate system that reduces the problem of inadequate heat extraction from the bulk layer enhances the problem of substrate layer porosity.
What is needed, therefore, is a substrate system for spray forming molten metal droplets to form a metal product that effectively solves both problems of substrate layer porosity and inadequate heat extraction from the bulk layer. Such a substrate system would serve to eliminate or significantly reduce both porosity in the substrate layer and reduce grain size and constituent particle size in the bulk layer by providing more efficient heat extraction rates to the bulk layer region.
The invention has met or exceeded the above-mentioned needs, as well as others.
A substrate system for receiving a deposit of sprayed metal droplets comprises a movable outer substrate on which the sprayed metal droplets are deposited and an inner substrate disposed adjacent the outer substrate where the sprayed metal droplets are deposited on the outer substrate. The inner substrate includes zones of differing thermal conductivity. Preferably, the zones of differing thermal conductivity include a low thermal conductivity zone, an intermediate thermal conductivity zone, and a high thermal conductivity zone. The low thermal conductivity zone is disposed adjacent the outer substrate at a location where a leading edge of the sprayed metal droplets are deposited on the outer substrate. Advantageously, this prevents chilling at the substrate layer and eliminates or significantly reduces substrate layer porosity. The high thermal conductivity zone is disposed adjacent the outer substrate at a location where a bulk layer of the sprayed metal droplets have accumulated on the outer substrate. Advantageously, this provides for rapid heat extraction from the bulk layer so as to prevent the development of coarse constituent particles and large grains in the product being produced. The intermediate thermal conductivity zone is disposed between the low thermal conductivity zone and the high thermal conductivity zone and provides a transition area therebetween.
A spray forming apparatus is also disclosed which includes an atomizing spray nozzle for creating a spray of metal droplets and a substrate system for receiving a deposit of the sprayed metal droplets. As described above, the substrate system includes a movable outer substrate on which the sprayed metal droplets are deposited and an inner substrate disposed adjacent the outer substrate and having zones of differing thermal conductivity.
A method of spray forming a molten metal to form a metal product is also provided. The method first includes atomizing the molten metal to produce sprayed metal droplets and then depositing the sprayed metal droplets onto a movable substrate such that porosity in a substrate layer of the metal product is resisted and further, such that grain size in a bulk layer adjacent the substrate layer is reduced. Preferably, the method provides for employing aluminum as the molten metal.
A further method of spray forming a molten metal to form a metal product includes atomizing the molten metal to produce sprayed metal droplets and then depositing the sprayed metal droplets onto an outer movable substrate overlying an inner substrate having zones of differing thermal conductivity.